Actually I don´t know what type it is. When saving these files, photoshop has just saved them without any extension, as this work on mac
I have tried to use .jpg .tiff etc on the pc, but this won´t work.

Are you sure they have no extension, or are file extensions for known file types turned off in Windows Folder Options?

Photoshop's default save format is usually a .PSD. If you have PS installed on Windows, try right clicking on it, picking Open With, and choosing Photoshop from the list (you might need to browse to it manually if it's not in the list).

If that doesn't do it and they really do have no extension, rename one of them to whatever.PSD and then double click it - if PS is installed it should automatically use it to open the file.

Photoshop 4.0 was from 1996, and 10.2 is utterly obsolete. You might as well be asking it to read an Appleworks floppy!

You're going to need to open -- every one of those 5,000 files on the old Mac and re-save them for PC compatibility (with extension and so forth). You should probably try to find someone with a more recent version of PS for the Mac, they may be able to do this as a batch job and save you a lot of time.

This job really should have been done a decade ago ... I'm not sure CS5 on the Mac could even open those files!